Using a nerve fibre half a millimetre wide and a clever circuit called the voltage clamp, Alan Hodgkin and Andrew Huxley worked out exactly how the action potential is made — and wrote it down as equations still used today.
History of Neuroscience
Advanced
Hodgkin, Huxley and the Squid Giant Axon
The Editors
May 19, 2026
3 min read
In plain English
Squids have unusually thick nerve fibres, wide enough to slide a wire inside. In the 1940s and 50s Alan Hodgkin and Andrew Huxley used them to measure the electrical currents of a nerve impulse and showed it comes from sodium rushing in and potassium rushing out. Their mathematical model won a Nobel Prize and is still the basis of neuron simulations.
Advanced level: The detailed machinery — ions, receptors, circuits and methods. Read the linked concepts first.
Concepts to know first
By the 1930s it was clear that nerve impulses are electrical, but not how they were generated. The breakthrough came from an unlikely animal.
A giant axon
In 1936 the zoologist J. Z. Young pointed out that the squid's escape system uses axons up to about a millimetre across — hundreds of times wider than most mammalian axons. A wire electrode could be threaded inside. In 1939, at the Plymouth marine laboratory, Alan Hodgkin and Andrew Huxley made the first recording of an action potential from inside a nerve fibre. To their surprise the voltage didn't just fall to zero during the impulse; it overshot, briefly becoming positive inside. The old theory — that the membrane simply became leaky — couldn't explain that. Then the Second World War interrupted, and both men worked on radar and gunnery.
The voltage clamp
After the war they used the voltage clamp, a feedback circuit (developed with Bernard Katz, building on work by Kenneth Cole and George Marmont) that holds the membrane at a fixed voltage and measures the current needed to keep it there. By swapping the sodium in the bathing solution, they separated the current into two parts: a fast inward flow of sodium ions that drives the rising phase, and a slower outward flow of potassium ions that brings the voltage back down. The sodium current also switches itself off — inactivation.
Equations for a nerve
In a famous series of five papers in 1952, they described these currents with a set of differential equations — the Hodgkin–Huxley model — and calculated, by hand-cranked calculator, the shape and speed of an action potential. It matched the real one. They had predicted voltage-sensitive "gates" in the membrane decades before ion channel proteins were isolated.
Hodgkin and Huxley shared the 1963 Nobel Prize with John Eccles. Their model remains the foundation of computational neuroscience.
Everyday examples & things to try
Analogy
The voltage clamp as cruise control
A voltage clamp works like cruise control on a car. You set the speed (voltage); when the road tilts (ions start flowing), the system adds or removes power (current) to hold the speed. Measuring how hard the cruise control works tells you exactly how strong the hidden forces are.
Real world
The squid's escape
Why does a squid have giant axons? Speed. Wider axons conduct faster, and the squid uses them to fire its whole mantle at once for jet-propelled escape. Vertebrates solved the same problem differently — with myelin.
Key takeaways
- The squid giant axon was wide enough for an internal electrode.
- The voltage clamp showed the action potential is a sodium influx followed by potassium efflux.
- The 1952 Hodgkin–Huxley equations still underpin neuron models today.
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